The present invention relates to a process for reproducing images of fine lines or characters in an electrostatic latent image transfer system, and more particularly to a process for reproducing images of fine lines or characters of low density which includes the steps of latent image transfer by electrostatic latent image transfer techniques and image development by wet type liquid developing techniques.
While various processes for transferring electrostatic latent images have been proposed in the past, such prior art processes may roughly be classified into two categories, namely, the transfer by an electrically grounded system and the transfer by application of a biasing voltage.
As a representative example of transfer by an electrically grounded type system, there is the process shown in Japanese Patent Publication SHO42-19757 published on Oct. 4, 1967. The process disclosed in that patent publication comprises, inter alia, the step of bringing a transfer paper, consisting of a dielectric layer coated over an electrically conductive layer into close contact with the surface of a photosensitive member on which an electrostatic latent image is formed. In this step, the transfer paper is brought into intimate contact with the surface of the photosensitive member and with relatively large pressure applied from the rear face of the paper so as to essentially provide no air gap between both surfaces. With essentially no air gap, the transfer of the latent image onto the transfer paper is enhanced by electrically grounding the base of the photosensitive member and the electrically conductive layer of the paper through an electroconductive roller.
Another example of image transfer by an electrically grounded system is U.S. Pat. No. 3,666,458 which discloses the process wherein the transfer of a latent image onto transfer paper is carried out by bringing the paper into virtual contact with the surface of the photosensitive member without application of any pressure; and by grounding the paper and the photosensitive member with the virtual contact maintained.
Another example of the former process is also described in U.S. Pat. No. 3,824,012 assigned to the same Assignee as this patent application. In this process which is illustrated in FIG. 1, an electrostatic latent image corresponding to an original 0 is formed on the surface of photosensitive member 1 in a well-known manner. That is, photosensitive member 1 is uniformly charged by corona charger 2 and subsequently exposed by exposure lamps 9 through mirrors 3, 4 and lens 5. Transfer paper P is in the form of a roll and consists of a high resistance dielectric layer (with a resistivity greater than 10.sup.13 .OMEGA./cm) coated over a high resistance conductive lining layer (with a resistivity in the range of about 10.sup.5 to 10.sup.10 .OMEGA./cm) and is initially brought into contact with the surface of photosensitive member 1, bearing the latent image, by an insulator member 6 in the form of a sheet. Insulator member 6 may also be in the form of a roll. Insulator member 6 is electrically insulated to have higher resistance than the conductive lining layer of the paper. Then paper P is passed between electrically grounded conductive roller 7, having a lower resistance than the conductive lining layer of the paper, and photosensitive member 1. A small air gap (normally at least 10 microns) is maintained between the surfaces of photosensitive member 1 and paper P to complete the transfer of the latent image onto the transfer paper P. More specifically, in this process, no strong electric field is generated between the transfer paper and the latent image during the approach of the paper onto the latent image as the electrical potential of the conductive lining layer of the paper positioned at insulator member 6 rises in accordance with the potential of the latent image. This effectively prevents the premature transfer of the latent image which is an inherent shortcoming in any of the latent image transfer processes. Thus, with this process, the transfer of the latent image is expedited within the range limited by the resistivities of insulator member 6 and grounded conductive roller 7 so that the high potential portion of the image is transferred during the passage of the paper about insulator member 6, with the transfer of the low potential portion of the image following thereafter being effected by grounded conductive roller 7.
Among the latent image transfer processes using electrically grounded systems described above, the last process described is most effective if a photosensitive member having comparatively large electrostatic capacity and which can be charged to relatively high potential is used. An example of such a photosensitive member comprises a photoconductive layer mixture of Se and As (or Se alone) having a thickness of less than about 1 micron disposed over a conductive base, with a polyvinylcarbazole layer of about 20 microns thick disposed over the photoconductive layer. The high potential charging is obtained because the charging of polyvinylcarbazole is as high as 50 to 70 volts per micron, which is about twice as high as a conventional single-layered Se photosensitive member. Accordingly, such a photosensitive member may then be made capable of accepting a high charge potential, which is necessary in the grounded transfer process, with the polyvinylcarbazole layer thickness as thin as about 20 microns. Thus, with an over-all thin thickness of the photosensitive member, its electrostatic capacity may be retained high to effect satisfactory transfer of the latent image, as may be well understood from the equation: Q = C V, wherein Q represents the amount of charge to be transferred, C the electrostatic capacity, and V the transfer potential.
However, if a single layered photosensitive member consisting of Se or a mixture of Se with As or Te is to be used in the grounded transfer process, the thickness of the photosensitive member should preferably be made thicker in view of the low charging retention characteristic of selenium, although this will cause the electrostatic capacity to become low. Accordingly, while such a photosensitive member may be used in the grounded transfer process by improving other characteristics contributing to the transfer of the latent image, such as the electrostatic capacity of the transfer paper, it is best to use such a photosensitive member in the transfer process in conjunction with a bias voltage application system as shown in U.S. Pat. No. 3,147,679, in which the image transfer is effected by the application of a biasing voltage to either the photosensitive member or the image transfer means.
To examine the reproducibility of an image corresponding to the original, experiments were conducted on some of the above described exemplary processes. The results obtained show that while such processes reproduce comparatively satisfactory copying images, the following disadvantageous phenomena were observed. Images of low density, particularly images of fine lines and/or characters of relatively low density in the original were not reproduced hardly at all. Foggy areal images not seen in the original were visualized on various parts of the transfer paper. Also, when fine images of low density were reproduced the foggy areal images were also visualized. Herein, the term "images of fine lines or characters", which corresponds to the low density portion of the original having such images, means fine images of low transfer potential. This definition should be regarded as applicable also to such terms as "images of fine lines or characters of low density", "fine images", "fine lines", "fine characters" or "line images" whenever used herein. Examples of fine images, i.e., images of fine lines or characters of low density are the slender lines often drawn in industrial or mechanical drawings which have thin thickness and low depth of color, or any characters or pictures of low densities represented by lines, particularly by lines of low density. The causes for these disadvantageous phenomena will be described hereinafter and there is a need to solve the problems caused by such phenomena.